1800-year Ocean Cycle Augments Warming

New research on long-term ocean cycles reveals rapid global warming in near
future

20 MARCH 2000 AT 17:00 ET US

Scientists at Scripps Institution of Oceanography at the University of
California, San Diego, report evidence of pronounced changes in the earth's
climate that can be tracked in cycles of ocean conditions over thousands of
years. These cycles reveal that Earth is currently in a period in which a
natural rise in global temperatures - combined with warming from the greenhouse
effect - will push the planet through an era of rapid global warming.

Charles Keeling and Timothy Whorf report in the March 21, 2000 online edition
of the Proceedings of the National Academy of Sciences (PNAS) that
strong oceanic tides are the engines behind this warming-cooling cycle that may
help determine future climates. This report is the first comprehensive study of
the effects of tidal mixing on climate change spanning millennia. The current
phase in the cycle suggests that a natural warming trend began a hundred years
ago, picked up in the 1970s, and should continue over the next five
centuries.

"We have discovered an 1,800-year tidal cycle that appears to match with
recent climate change," said Charles Keeling, the study's first author. "If this
is a correct mechanism for understanding climate change over millennia, then
temperatures will rise both because of weaker tidal mixing and because of the
greenhouse effect, which is on the increase as well."

The researchers suggest that strong oceanic tides drive changes in climate
due to their ability to increase vertical mixing in the ocean and thereby
transport cold ocean water to the surface. The strong tides elicit cool
conditions on the sea surface, which in turn lowers temperatures in air and over
land, resulting in cooler climates around the planet, often accompanied by
drought conditions. Weak tides lead to less cold water mixing and result in
warmer periods on Earth.

Keeling and Whorf's 1,800-year cycle, which arises because of gradual changes
in the astronomical alignments of the sun, moon, and earth, was proposed as an
explanation for nearly periodic millennial changes in temperature seen in ice
and deep-sea sedimentary core records. Previously (1997), they have reported on
the effects of shorter cycles of tidal forcing on global temperature at periods
near 18, 90, and 180 years.

A maximum in tidal cooling near 1974 might have produced more cooling, but
perhaps was masked by a simultaneous greenhouse warming, according to Keeling, a
professor of oceanography at Scripps.

"If that is true, then it becomes pretty clear that if today's natural
warming trend is combined with the greenhouse effect, then we'll soon see the
effect of combined warming all over the world," said Keeling. In addition to
climate change recognition, the research also represents a new mechanism for
analyzing events in world history.

The paper reports on the near coincidence of major tidal fluctuations with
worldwide phenomena, including the Little Ice Age of 1400 A.D. to 1700 A.D.,
major dust layers in Minnesota lake sediments spaced about 1,800 years apart, a
major drought in the Amazon Basin around 2200 B.C., and a 2000 B.C. drought that
may have contributed to the collapse of Akkadia, a Mesopotamian civilization
regarded as the world's first empire. The Vikings inhabited Greenland in
temperate conditions in the tenth century near the end of a period of weak tidal
activity, but perished or left Greenland when tides strengthened near the
beginning of the Little Ice Age in the 13th century.

"One of the principle benefits of the tidal hypothesis is that researchers
can compare the timing of specific historical events with predicted times of
warming or cooling to see whether they coincide or not," said Whorf, a research
associate in the Geosciences Research Division of Scripps. "If we are correct,
then the 1,800-year tidal cycle will be important in understanding future
climates as well as events of the past."

The study was supported by the National Science Foundation and the U.S.
Department of Energy.

A warming trend in the planet’s oceans may point towards a warmer future.

The ocean has long been known as the incubator of life, but now some
scientists hypothesize it may play another important role by regulating global
climate changes.

The two scientists, Charles Keeling and Timothy Whorf, report in the
Proceedings of the National Academy of Sciences (PNAS) that strong oceanic tides
drive the planets long-term temperature cycle.

The pair, working at the Scripps Institution of Oceanography at the
University of California, San Diego, say they have found pronounced changes in
the warming and cooling cycles of the ocean which take place over thousands of
years.

These cycles suggest that the earth is currently in a natural warming period
which began about a hundred years ago and should continue over the next five
centuries.

"We have discovered an 1,800-year tidal cycle that appears to match with
recent climate change," said Charles Keeling, the study's first author.

The two researchers say that this cycle is related to gradual changes in the
astronomical alignment of the Sun, Moon and Earth, which influences the tides.
In simplest terms, when the tides are strong it increases vertical mixing in the
ocean, bringing cold water to the surface and thereby bringing cooler conditions
to the planet’s surface.

Weaker tides, in contrast, bring less cold water to the surface and result in
warmer periods on Earth. Keeling and Whorf say records of ice and deep-sea
sedimentary core samples show evidence of the cyclical changes in the Earth’s
temperature.

These records match climatic events in world history, such as the Little Ice
Age of 1400 A.D. to 1700 A.D., which may have forced inhabitants of Greenland to
abandon their homes, or the drought in Mesopotamia around 2000 B.C., which
likewise may have contributed to the collapse of the ancient empire of Akkadia.

"One of the principle benefits of the tidal hypothesis is that researchers
can compare the timing of specific historical events with predicted times of
warming or cooling to see whether they coincide or not," said Whorf, a research
associate in the Geosciences Research Division of Scripps. "If we are correct,
then the 1,800-year tidal cycle will be important in understanding future
climates as well as events of the past."